(a) Field of the Invention
The present invention relates to a structure and a method for manufacturing a semiconductor optical waveguide and, more particularly, to fabrication of an improved optical waveguide for achieving a high coupling efficiency with an optical fiber by forming a circular and narrow optical beam in a semiconductor laser or a photodetector.
(b) Description of the Related Art
The mode field of an optical signal from a semiconductor laser device, for example, should be adjusted to the mode field of an optical fiber to be coupled for obtaining a high coupling efficiency as much as possible. A mode field converter (MFC) is generally installed for this purpose in the semiconductor laser device.
FIG. 1 is a cross-sectional view of a conventional product of a semiconductor laser device having a MFC, and FIGS. 2A and 2B are cross-sectional views thereof taken along lines I-I' and II-II' in FIG. 1. FIGS. 3, 4A, 4B, 5A and 5B show the semiconductor laser device of FIG. 1 in consecutive steps of fabrication process therefor, wherein FIG. 3 showing a first step thereof corresponds to FIG. 1, FIGS. 4A and 4B showing a second step correspond to FIGS. 2A and 2B, respectively, and FIGS. 5A and 5B showing a third step correspond to FIGS. 2A and 2B, respectively. The structure and the fabrication process for the conventional semiconductor laser will be described with reference to these drawings.
In general, a semiconductor laser device having a MFC section is fabricated by an epitaxial growth process using a low-pressure MOCVD (metal-organic chemical vapor deposition) method from the viewpoint of process simplification. In the fabrication process, first, a SiO.sub.2 film is deposited on a n-type InP substrate (n-InP substrate) 101 by using a plasma-enhanced CVD technique. The SiO.sub.2 film is then selectively etched by a photolithography and a wet etching technique using a BHF (buffered hydrofluoric) solution to obtain a plurality of stripe SiO.sub.2 patterns 120 shown in FIG. 3. Each of the stripe patterns 120 has a rectangular shape which is 800-.mu.m long (L) and 60-.mu.m wide (W), and each two of the stripe patterns 120 form a stripe pair with the distance (d) therebetween being, for example, 10 .mu.m. The stripe pairs are arranged in a matrix, with a gap (D1) of 300 .mu.m in the column direction and a pitch (D2) of 250 .mu.m in the row direction. Each of the stripe pairs and the area adjacent thereto in the column direction is formed as a semiconductor laser device, and accordingly, FIG. 3 shows an area for a plurality of semiconductor laser devices formed in a single process.
After the stripe SiO.sub.2 patterns 120 are formed on the substrate 101, as shown in FIG. 4A, an n-InP cladding layer 102, an InGaAsP/InGaAsP quantum well active layer 103 and a p-InP cladding layer 104 are consecutively grown on the exposed surface of the n-InP substrate 101 not covered by the stripe SiO.sub.2 patterns 120. In this epitaxial step, thick epitaxial layers 102 to 104 are formed in the belt area 124 (FIG. 3) disposed between each stripe pair, as shown in FIG. 4A, whereas thin epitaxial layers 102 to 104 re formed in the other area, as shown in FIG. 4B.
After the stripe SiO.sub.2 patterns 120 are removed by a BHF solution, a second SiO.sub.2 film is deposited on the entire surface by a plasma-enhanced CVD technique. Thereafter, the second SiO.sub.2 film is patterned using a photolithography and a wet etching technique to leave a belt SiO.sub.2 film 121 on each 4.0-.mu.m-wide belt zone defined by the belt areas 124 arranged in a column direction and the spaces between the adjacent belt areas 124 arranged in the column direction. A wet etching is then performed using the belt SiO.sub.2 film 121 as a mask and bromomethanol as an etchant to selectively remove the n-InP cladding layer 102, the quantum well active layer 103 and p-InP cladding layer 104, as a result of which 1.5-.mu.m-wide mesa stripe 123 is left below the 4.0-.mu.m-wide belt SiO.sub.2 film 121, as shown in FIGS. 5A and 5B.
Subsequently, blocking layers including p-InP layer 105 and n-InP layer 106 are laminated on the side surface of the mesa stripe 123, thereby embedding the mesa stripe 123 by using a MOCVD method as shown in FIGS. 2A and 2B. Thereafter, the belt SiO.sub.2 film 121 is removed using a BHF solution, followed by a MOCVD process to form consecutively a p-InP cladding layer 107 and a p-InGaAs contact layer 108. Next, the p-InGaAs contact layer 108 in the upper part of a MFC section II' is selectively removed by a photolithography and a wet etching technique using a tartaric acid based etchant, the MFC section II' being shown in FIG. 2B.
Then, a third SiO.sub.2 film 122 is deposited by a plasma-enhanced CVD process, and patterned to have an opening for an electric contact to be used for injection of carries in a laser section I' as shown in FIG. 2A. Thereafter, the n-InP substrate 101 is polished at the bottom surface thereof to reduce the thickness thereof down to about 100 .mu.m, followed by formation of p-side electrode 109 and n-side electrode 110 on the top surface and the bottom surface, respectively, of the resultant wafer, to obtain the structure shown in FIGS. 1, 2A and 2B.
In the conventional semiconductor laser device as described above, the cladding layer 102 and the laser active layer 103 have smaller thicknesses in the laser section I' than in the MFC section II'. By this configuration, a narrow and excellent optical beam can be obtained from the MFC section II' because of the smaller optical confinement area of the MFC section II'. In this case, because the MFC section II' is transparent for laser light, the optical loss is small in the optical transmission.
For the conventional semiconductor laser device having a MFC section as described above, a complicated process is required to form the optical waveguide therein. In addition, since the waveguide does not have a current confinement function in the direction of the resonator of the laser device, there arise a problem in that the carriers supplied to the laser section leak tat the MFC section to raise the threshold current for the lasing of the laser device.
In the case of the above described semiconductor laser device, the n-InP substrate used therein requests a p-InP epitaxial layer as the top layer for the layer structure, wherein the carrier leakage is effected by holes (not by electrons), which fact reduces the carrier leakage compared to the case wherein a p-type substrate is used and thereby the carrier leakage is effected by electrons. If a p-type substrate is used instead in the above laser device, the carrier leakage effected by electrons raises a larger problem.